Development of group III nitride based electronic and optoelectronic devices with high efficiency and reliability depends on many factors, such as a quality of the semiconductor layers, active layer design, and contact quality. In particular, designing highly conductive p-type gallium nitride (GaN) and/or aluminum gallium nitride (AlGaN) is important for a number of electronic and optoelectronic devices, including ultraviolet light emitting diodes (UV LEDs). Achieving a high p-type conductivity of magnesium (Mg)-doped AlGaN has been difficult due to a large acceptor activation energy of 150-250 milli-electron Volts (meV), as well as due to a low hole mobility in heavily Mg-doped AlGaN alloys. The problem is particularly severe with increased molar fraction of aluminum due to a further increase of the acceptor activation energy and also due to an increase in unintentional donor concentration with the increasing aluminum molar fraction. For AlGaN layers having high aluminum molar fractions, the oxygen (O) donor concentration can result in insulating or even n-type characteristics of the AlGaN layers despite heavy Mg doping.
Additionally, heavy Mg doping can negatively affect the reliability of the optoelectronic device. The existence of degradation beyond device self-heating has been previously observed and attributed to the migration of Al atoms from the p-type cladding.
One proposed degradation mechanism for group III nitride based LEDs is electrons with high kinetic energies crossing the p-n junction, thereby causing a decrease in output power. This energy is transferred into the lattice, and more specifically to the electron blocking layer designed to confine electrons within the quantum wells of the active layer. The energy released by electrons assists in breaking both Mg-hydrogen (H) bonds, further activating carriers in the p-type layer, and Ga—N bonds, creating nitrogen vacancies, VN. The increased Mg activation causes an initial rise in output power before reaching a steady-state, while the VN formation takes significantly longer to reach equilibrium and is responsible for the slow decrease in emission over a longer period of time. Alternatively, released electron energy may contribute to formation of Mg—H2 complexes and result in an overall decrease of p-type doping. The energy of formation for the nitrogen vacancy in p-type AlGaN has been calculated to be significantly lower than that of p-type GaN. However, the Mg—H2 complex is more stable in AlGaN than in GaN. Thus, in high-Al content devices, almost all of the atom displacement leads to VN formation, causing the slow further degradation observed in UV LEDs, which is manifested in an increase in the depletion edge on the p-side of the junction, which has been observed in the capacitance-voltage data, and which further shows that this behavior is amplified at higher current densities and associated operating temperatures.
Formation of nitrogen vacancies and other defects due to electron-lattice interaction results in effective trapping of holes in semiconductor layers. One approach to reduce degradation of semiconductor layers is through the use of a micro pixel device design, or by using LEDs with large planar area devices, which allow for reduced current densities and operating temperatures, limiting the velocity of electrons approaching the p-n junction, electron blocking layer, and p-type layer.